Remove useless check of AR being defined from teardown fixtures

We are already in the AR namespace, there's no way for it to be
undefined. See the cousin commit 13e72db7

Refactor a bit teardown fixtures to avoid two conditionals.

activerecord/lib/active_record/fixtures.rb

@@ -872,11 +872,7 @@ def setup_fixtures
     end
 
     def teardown_fixtures
-      return unless defined?(ActiveRecord) && !ActiveRecord::Base.configurations.blank?
-
-      unless run_in_transaction?
-        ActiveRecord::FixtureSet.reset_cache
-      end
+      return if ActiveRecord::Base.configurations.blank?
 
       # Rollback changes if a transaction is active.
       if run_in_transaction?
@@ -884,7 +880,10 @@ def teardown_fixtures
           connection.rollback_transaction if connection.transaction_open?
         end
         @fixture_connections.clear
+      else
+        ActiveRecord::FixtureSet.reset_cache
       end
+
       ActiveRecord::Base.clear_active_connections!
     end